An oscillometric blood pressure measurement apparatus (e.g., see Patent Documents 1 to 3) is known as an apparatus that automatically measures a body's blood pressure value.
An oscillometric automatic blood pressure measurement apparatus slowly changes the pressure with which a cuff wrapped around a part of a body pressurizes the body (cuff pressure) at a predetermined speed, and detects the pressure in the cuff while the pressurizing pressure is being changed. Also, a pulse wave, which is a pressure component that is superimposed on the cuff pressure in synchronization with the body's pulse, is detected in the detected pressure in the cuff, and the body's blood pressure value is determined based on the change in the amplitude of the pulse wave.
Patent Documents 1 and 2 disclose automatic blood pressure measurement apparatuses that detect a pulse wave by passing a cuff pressure detection signal through an analog high-pass filter or a digital high-pass filter and calculate the blood pressure value based on the amplitude of the detected pulse wave.
With this type of automatic blood pressure measurement apparatus, rapid change occurs in the cuff pressure detection signal in a process of changing from a step of increasing the cuff pressure to a step of reducing the cuff pressure. For this reason, in Patent Documents 1 and 2, a transient response period occurs in the output of the analog high-pass filter or the digital high-pass filter due to this change, and during the transient response period, the amplitude of the pulse wave cannot be detected accurately.
Note that if a high-order filter is used or the filter is constituted by multiple stages, the transient response period can be shortened to the extent that the accuracy of measuring the blood pressure is not influenced. However, high-order filters and multi-stage filters are high in cost.
In view of this, in Patent Document 3, in order to prevent a decrease in the blood pressure measurement accuracy and a decrease in the blood pressure measurement speed due to the transient response, a transient response period generated in the digital high-pass filter during a switch from increasing to decreasing the cuff pressure is eliminated by inputting a ramp signal into the filter and thereby resetting the filter.